About science; About radio telescopes
This is about science produced by the California Institute of Technology and originally broadcast by station KPP C. Pasadena California. The programs are made available to the station by national educational radio. This program is about radio telescopes with host Dr. Albert Hibbs and his guest Dr. Alan Moffat associate professor of radio astronomy. Here now is Dr. hit with a science of astronomy is probably about the oldest one in the world. And since men have been gazing and thinking about the stars the concept of the star has gone through quite a bit of evolution from a point of light on a crystal in spirit to. Some burning objects like the sun some distance out in space until finally we recognize a universe full of galaxies. But it's been rather recently that we've discovered they not only give the visible light that man of information was since they were man at all to look. They also give other kinds of radiation in particular radio signals or at least
radio noise that comes from stars and from space. There have been since this discovery a long series of bigger better and more accurate instruments built to observe this radio signal that comes from space and from the stars to try to deduce what it means. And in the process in the process of this a considerable amount has been learned about the stars it simply was not known when only visible light was being observed. We have with us today one of the scientists who is intimately concerned with this and deeply involved Professor Alan Moffatt who studied at Wesleyan University and then at the California Institute of Technology where he received his Ph.D. Abel even 1961. He was a Fulbright Scholar and Bonn Germany in 1962 and then on the faculty of Caltech. He's been on the staff of the Owens Valley radio observatory where Caltech has their
principal. Radio astronomy installation. And perhaps to put this whole thing in a proper framework and I should start out by asking you when did radio astronomy begin as such. When did we first find out that the stars were giving us something besides light. We can fix that date quite precisely. It was in 1931 when an American engineer named Carl Jensen who was working at the Bell Telephone laboratories was a saying the problem of determining the minimum noise level that one could achieve in a radio telephone circuit operating at short radio wave links and he found that this minimum noise level was determined not by noise of terrestrial origin or even by noise from the sun but by noise originating in our own Milky Way galaxy. And this dawned on him in the course of his experimental investigation in 1931 and was reported in the years following that. So 1931 was the first time anybody had ever actually observed that there was a radio
some sort of radio energy coming from space. Yes general good did he at that time identify what it was that was doing this or just a general location in the sky. Well as his experiments progressed through the early 1930s he finally came to the definite conclusion that radio emission was coming from our own Milky Way galaxy and was strongest in the direction of the center of our own galaxy. So is this since been tracked down to associated with the mass of stars as it is at the center of the galaxy. Yes our get out of the radio emission from our galaxy has been studied extensively. There are several components of this mini emission none of them come from directly at least from the stars which we see in abundance with the naked eye or with the telescope the radio emission comes from the tenuous gas which exists in between the stars. I see well then in your own work is this. Is this the type of study that you have been following the radio emissions in the galaxy or if you've been looking at other sources.
My own work and for the most part the work of our observatory has been devoted more to extragalactic radio astronomy to studies of other galaxies objects much farther away than any of anything within our own galaxy which for one reason or another emit very strongly in the radio wavelength range. What sort of equipment do you need for this as an optical telescope of course has a large glass object either a lens or a mirror at one point and then the photographic film to pick up the result of the other maybe a spectroscope. What. Sort of object you to look for radio wave just a piece of wire stretched out across between two poles so that's one kind you can in fact make studies of celestial radio emission with a simple wire antenna particularly at the longer wavelengths the antenna which chance he used was not too different in principle or somewhat different in appearance from the kind of beam
in tennis that radio amateurs use or the sort of antennae you used to pick up a distant television station at the shorter wavelengths we use antennas which more closely resemble a large reflecting optical telescopes in that they consist of a large parabolic mirror. In this case made of either a metal mesh or a metal sheets which have been stretched to a parabolic form. I see sometimes a mirror in the sense that a reflects or radio waves the same like it reflects the radio waves in the same way that the mirror in the 200 inch telescope for instance reflects optical waves the focal ratio of the rake at the ratio of focal length to de emitter is usually smaller for a radio telescope so that the focus is. Less than one day in a matter of the reflector away from the bottom of the reflector what happens with the waves when they get reflected to put the just ordinary receiver then mounted someplace to pick up this recent and so waves are reflected and brought to a focus and at the focus instead of a photographic film. There is a radio
receiver a small antenna which picks up the focused energy from the reflector and leads this energy to a receiver which is not different in principle from a television receiver or the front stages of a television receiver where these energy is amplified and oftentimes converted to a lower frequency and then amplified some more and eventually brought to some sort of an indicator oftentimes a recording. A meter which draws a line on a piece of paper which moves slowly into the pen. I say you are then so that it's a signal that you're made this for the strength of the signal you measure than the usual iconic ways in the same way that you might measure the strength of a signal with a spectroscope or a photographic image or an optical. Yes often times one puts an energy receiver of a similar nature in an optical telescope by a photo tube which picks up which delivers a signal proportional to the energy which is incident on its front surface. In our radio telescope receivers oftentimes behave the same way we have an output signal on
the recorder which is proportional to the amount of energy in sit in and the big reflector. Well the largest optical telescope has a reflecting surface 200 inches in diameter and so on a polymer How big are the reflecting surfaces of radio telescopes. Well they vary. The largest telescope of a reflecting variety is the thousand foot telescope at RC Boeing Puerto Rico which is so large that it is built into a natural sinkhole a natural valley shaped more or less like a spherical Bowl. A good deal of tailoring was required to make the valley into a good sphere and within this there is a wire mesh reflector which is in the form of a thousand foot section of a sphere a cap of a sphere. And then at the focus of this here there is a rather complicated antenna in this case which picks up the radiation brought to a housing antenna howled over this thousand foot. This is quite a complicated engineering
job there is a largish triangle. Looks like a piece that is was stolen from a rather large bridge which is held out over this great belt 800 feet above the bottom of the bowl by. Three sets of four heavy steel cables and or is it six heavy steel cables each one about the size of my wrist and assembling this thing in mid-air was quite a feat. I magine it was a gigantic bridge building job that my far above the ball below made a lot of the bridge builders already making radio telescopes because they are indeed gigantic structures. But what about the ones that you see pictures of these spidery looking dish objects that are pointed up in the sky. How big do those get these ones that tilt and turn. Yes these are actually pointed directly at the object the telescope. The reflector is fixed in the pointing over a limited range of angle is done by moving the receiving antenna the more common type which one has seen in
pictures. Quite frequently no doubt is. Pointed to different parts of the sky by actually physically pointing the whole reflector and receiver to the direction from which one wishes to receive the radiation. The largest of these is 300 feet in diameter. However it is movable in only one one coordinate it always points at the local meridian the line passing from north to south straight overhead. And so it can see any given object only once per day as the object passes over that line. And the very are other telescopes and many others which are movable in both directions and which can be pointed more or less to any part in the sky. The biggest of these are still with two hundred fifty foot telescope at Jodrell Bank in England which was built more than ten years ago now. So that then these are ones that are movable in the sense of the usual optical telescope run in the range of a couple of hundred feet versus for the optical and a couple hundred inches. Yes 12 times as big something to watch. Why the
large size Is there something special about radio waves that makes it necessary to go so big. Yes they are very very weak. I think I had my students this year calculate what the total amount of energy is that falls on the earth from our radio sources in all of the sky. It's about one and a half kilowatts just about enough energy to brown a good piece of toast falling on the whole works whereas light energy from the sun is about that about that much about 1 kilowatt perspire meter. I see what about the light energy from the stars how does it compare with all of the incident in your calculation take into account the radio energy from the sun to know that two that exploded the sun how do how does starlight compare with radio light from the stars you have any estimate of the difference between those two power levels. Any guess that I'd make would be pretty far off let's see. I'm leading you off into an unfair piece of a rest. Yes I know you said if you take a shower at night it's certainly the energy density of
starlight is. It's somewhat faint stars are somewhat smaller than the energy density of radio emission. But the detectors are of a quite different nature and it's difficult to make the comparison anyway in order to detect faint radio sources we need exceedingly large radio telescope was the size of the size the reflector then is based upon the nature and strength of the kind of signal you're trying to get from these things. Yes you can detect fainter absolute signals in the optical range because the individual quanta of the individual photons which compose the signal are energetic enough to be detected individually in the radio range. This is not so. On the other hand in the radio range the method of detection is different. We can amplify the radio signals or as amplification of light while possible is not as efficient certainly not as well developed in art but I know it is a Radio One. When you mention before you were looking for your own work was based on a search for
or examination of extra galactic radio sources. What are the extragalactic radio sources what do they consist of. Well there are two kinds. Three really. There are normal galaxies galaxies like our own which radiate at radio frequencies largely due to radiation from cosmic ray particles which are in captured and held within the galaxies by their own magnetic fields. This is the type of radiation which chance be detected from our own galaxy and naturally other galaxies like ours have similar radiation. Is this a principle thing you see then and when you look out for extra galactic sources know these these radiate fairly weakly and we can only detect the nearest normal galaxies. You know addition to these there are peculiar radio galaxies we call them because they have a peculiar because they have very much enhanced radio emission. Factors of ten thousand to a million times more radio emission than normal
galaxies. Sometimes even hundreds of million times more than normal galaxies of their own type. And for some reason or other a great amount of energy has been released within these galaxies and produces clouds of energetic electrons which radiate the radio signals which we can detect from these objects. How can you tell for exactly what it is you're looking at when all you can see is a radio signal. How do you know that it's a galaxy for example. Well this is a rather complicated procedure. The first thing which we do when we want to study radio source is to measure very carefully its position in the sky. We also measure the strength of the radio emission coming from it at different frequencies. The diameter of the region which gives rise to the radio emission the angular diameter In other words its shape if we can and other properties of the radio emission. But then using the accurately determined radio position we search for an optical counterpart. This is done by by examining
what we find in that position on a survey of the entire sky which was made with the camera at Mount Palomar. And quite frequently we find some sort of way an image of a galaxy in close agreement with the position of the radio source. So it's a comparison that of optical observation on the radio that rely on to identify the object. Yes sometimes we have a strong hint of what kind of object we're looking at from let's say the shape of the radio source. I suppose however the hint is based on considerable knowledge and experience with us only after we've examined a lot of these objects do we know what kind of common characteristics they have recognize a signature. Yes. Is there anything special about the galaxies that seem to be strong radio sources. Well they own seem almost all of them optically have evidence that some sort of a disturbance has gone on within the galaxy. It's not very common for ordinary galaxies to have strong
emission lines to emit a strong spectral lines in the optical range from highly excited atoms of oxygen and neon hydrogen. They're mostly what absorption lines is and yes you really the spectrum of the galaxy shows a rather broad absorption lines just from the spectra of all the stars in the galaxy which are what's giving rise to the light after all. But in these radio galaxies some sort of an explosion has occurred giving rise to the radio source and leaving behind a residuum of disturbance which produces these optical emission lines this is fortunate because it is easy to measure the redshift the Doppler shift. The U.S. mission lines in the optical spectrum of these galaxies and from the optical redshift one gets a measure of the distance of these objects which would otherwise be hard to obtain. Unfortunately there's no way a way to measure the distance to a radio source from radio
observations alone there is no set of strong emission line in the radio spectrum which we could use can't peg a distance of course is obtained from the redshift by a use of Hubble's Law of the law which says that the farther away something is the faster it seems to be receding from us as it takes part in the general expansion of the universe. So then by identifying the galaxy it's coming from and checking the spectrum of the galaxy you can find of how far it is what about the Israeli and also the emission lines say that there's some things happen in the galaxy is there anything else about the Galaxy besides the evidence of an explosion or some activity where they are they galaxy like the Milky Way for example that you see occasionally putting out the strong radius or most of the strong radio galaxies are of a different type from the Milky Way there are what are called elliptical galaxies the Milky Way is a spiral galaxy. We have strong evidence of the spiral arms and we see many other galaxies which
must look quite like our own although we can't get a bird's eye view of our own galaxy. The spiral galaxies don't seem to very often produce strong radio sources. The strong radio sources come from other galaxies which are often times even more massive than our own galaxy which is a very large galaxy but which don't have the characteristic spiral arms they're more of an amorphous spheroidal cloud of stars and usually don't contain very much interstellar material. One can see right through them sometimes but somehow the ones which have turned into radio galaxies have had a release of sufficient gas between the stars in these galaxies so that the emission lines can be generated we can detect them. Is there any radio Sir are there any radio sources so far that you've been able to pick up what you can't identify with a particular galaxy or some optical source. Yes the strongest radio sources.
Are nearer to us just they seem to appear stronger simply because they are near and the radiation hasn't had to spread out over so much volume and all of the strongest radio sources have been identified with optical counterparts. But as one moves to fainter and fainter radio sources a increasingly larger fraction cannot be identified with any object which can be seen on the Palin Marchment Sky Survey. Did you suspect this is just because they're further away. We think so that they are not intrinsically different from the identified objects which are nearer to us. They're simply so far away that while we can detect their radio emission at least with the problem Marchment telescope which is surveyed the whole sky we can't see their optical emission. So this means that the large dishes on the radio telescope of outstripped the optical ones in some sense they can see things the optical ones can't. Yes we think that many of the radio sources which are detected at the faintest threshold of existing
instruments are so far away that their optical emission cannot be detected. So this is really the first one of the furthest probes into space that by means of the radio we think it is. There's another source of course to become quite famous lately the quantized star radio sources. Do you have any feelings about these as quite a controversy that I've heard about over the last few months as to whether or not there are really very distant objects as they've been thought of whether they're perhaps close but just moving fast. How do you stand on this and what do you think the evidence shows. Yes perhaps we should explain a little more about what these are these are another category of apparently extragalactic radio sources. Almost all of them quite small meaning it are size which alone if they were similar to radio galaxies would indicate that they were very distant. However they don't seem to be very exactly like radio galaxies. The optical counterparts of these objects look like stars on photographic plates that's why they're called quasi stellar radio sources. We
found the first two or three of these from positions that were measured with our instrument in the Owens Valley. And when we examined what we might see in the position of the radio source there was nothing but a star and the spectra of these objects the optical spectra showed us that these really weren't stars but were. Unknown an unknown type of object something that had never been detected before even though there were on the plates it just had not had any characteristics optically that made them. Yes there are billions of feet stars on the photographic plates and one has to do everything mentioned run to unusual ones well these are unusual enough all right. And when the key to their spectra was provided a year or so later it was clear that all of these had very large red chips the nearest of them has a redshift as great as were quite a faint galaxy and yet it's almost a hundred times brighter as a galaxy with a comparable redshift. So they're moving away from moving away from us. That Percival
fraction of the speed of light. Yes up towards 80 percent of the speed of light for the farthest ones. And this if if they are really participating in the general expansion of the universe of course this means that these guys the stellar radio sources are by far the most distant known objects in the universe. I see. And yet there is as I've heard some believe that maybe they're not maybe they're closer but just moving faster. Well the reason for this belief or doubt in the belief that they are distant objects is that while every luminous objects if they are at these distances their apparent intensity is tell us that there are perhaps 10 to 50 or even 100 times more luminous than the brightest known galaxies. And yet these things are small enough physically so that their light intensity can vary appreciably in a few days in the optical wavelength range and in times as short as a few months at short radio wavelengths or a
few years at medium length radio wavelengths. And to us this implies that the emitting region or at least a region which gives rise to an appreciable fraction of the emission from these objects is not more than a few light days in the optical case or a few light months or years in the radio case in diameter. Because if the object were larger. One wouldn't see variations even if a given part of the object had short time variations. The mission from other parts of the object which were further removed would appear out wash out any variation and you do see these vary so all these variations seem to be a common property of the car as a starter. Radio sources particularly in the light range they don't all bury in the radio range of wavelengths but almost all of them vary at light wavelengths so the sun implies a small size. They're very small in size and it is very difficult to conceive of a mechanism which could produce such very high luminosity such a very large power output from such a very small volume. I see so it is for this
reason that people have suggested that perhaps these objects are not really so distant after all which would reduce the required power output but leaves one with a very embarrassing task of explaining their very large velocities the very large velocities with which they seem to be moving away from us. And part of the task that you have in identifying both the nature and the position of. The radio sources you can use more than one telescope at a time can't use of is not possible to hook up a couple of telescopes and use them in combination to get an interference pattern and help locate both position and get more information about the radio sources you're looking at. Yes Being get a resolution of radio telescope is determined by the size of the telescope its diameter divided by the wavelength to which it operates and the existing large single antennas of a few hundred feet in diameter are just
about as large as one can build single antennas and these have angular resolutions. A few minutes of arc. It's best to stop at 10 or to and identify the diameter of the moon. Somewhat poorer in fact than the angular resolution of the naked eye. And yet we need to do to resolve details in resources of radio mission which are small or anythingit or diameter in this. So it's necessary to use other techniques in order to work on the smaller objects and the techniques have been used in the general class of interferometry where one separates his telescope into a number of pieces and distributes them about the landscape with much greater distances between them than the diameter of the largest single telescope that you can build and by suitably processing the signals which are picked up at the same time by these different telescopes. One can obtain some of the fine resolution that one would like.
This is something you can do with a radio telescope it can with an optical because the power of electronics to combine the signals. Yes. You can't conduct light signals very easily over coaxial cables whereas you can conduct radio signals over a coaxial cables is about what that amounts to. What sort of a setup you have of the on's Valley for this. Well for several years now since 19 60 about we've had two 90 foot telescopes which we use as an interferometer and which we have been able to use to resolve details in radio sources down to perhaps a half a minute of arc. And is this what's the separation between the two. The maximum separation that we have at the moment is about sixteen hundred feet either in east west or in the north south direction or some combination of those direction and the telescope diameter itself and the two telescopes are each one is 90 feet in diameter and I see the 600 foot that has a tremendous advantage Yes just the diameter of the telescope alone. You see already the 600 feet is larger than the largest single reflector which is the thousand foot reflector in Puerto
Rico. Well as it is of any value to put more than two yellow scopes in an array like this the more you have the more information you get. Per unit time. So if we had three telescopes we would get three combinations of spacings whereas with two we only get one combination of spacings. Is there any possibility that you can increase your numbers of instruments there at Owens Valley. Yes we have this advantage we've developed a plan which we would like to follow through on which would result in a very fine instrument that would have 8 130 foot diameter telescopes with sufficient track on which to move them so that we can separate them by 9000 feet in the east west direction or even 16000 feet in the north south direction. And this would give us an instrument which could make a picture of a radio source with the resolution of finer than 10 seconds of arc in the course of a single day. So in this way you might be able to get much more information about exactly what portion of a galaxy exactly we could make much
finer and more detailed pictures of these extra galactic radio sources for instance. Incidentally we could also make pictures of some of the objects which are nearer to us and more familiar to us. Jupiter for instance the planet Jupiter has quite a strong radio source about it caused by the electrons which are captured in Jupiter's Van Allen belts they're very much stronger than the earth's been around belts but must be quite similar. And while we've made some rather crude pictures of this. Shallow of energetic particles around Jupiter we like to make much more detailed ones so it's not only these very remote objects which are interesting objects for study but in both cases the increase of resolution is all now almost a key to learning anything more. Yes I think so. I don't know what's going on well it sort of. I certainly wish you luck with the investigator with the possibility of getting more and so improving your resolution and your ultimate knowledge of or all of this energy is coming from that you've been watching so diligently all these years. And thank you thanks very much Alan for being
- About science
- About radio telescopes
- Producing Organization
- California Institute of Technology
- KPCC-FM (Radio station : Pasadena, Calif.)
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- University of Maryland (College Park, Maryland)
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- This program focuses on the use of radio telescopes for space research. The guest for this program is Alan Moffet.
- Interview series on variety of science-related subjects, produced by the California Institute of Technology. Features three Cal Tech faculty members: Dr. Peter Lissaman, Dr. Albert R. Hibbs, and Dr. Robert Meghreblian.
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Guest: Moffet, Alan
Host: Hibbs, Albert R.
Producing Organization: California Institute of Technology
Producing Organization: KPCC-FM (Radio station : Pasadena, Calif.)
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University of Maryland
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